Frequency Division Multiple Access (FDMA) is a communications technique that divides a frequency band into a plurality of sub-bands, i.e., channels. Each of a plurality of communication devices is assigned one of the channels. Each of the devices has a signal generating circuit that generates a preamplified communication signal within its assigned channel. Each of the devices amplifies the preamplified communication signal with a transmitter power amplifier that feeds the amplified communication signal to a transmitter antenna. The plurality of communication signals from the plurality of communication devices will not interfere with one another provided each stays within its channel band. However, transmitter power amplifiers can have non-linearities, meaning the output may not be linearly proportional to the input. The non-linearity can create distortion in the amplified signal transmitted. The distortion can appear as signals outside of the channel, which can extend into one or more of its adjacent channels, an effect termed “adjacent channel interference.” Costs can include communication signals degrading adjacent channel quality, as well as waste of amplifier power.
There are conventional techniques directed to reducing adjacent channel interference, but all have various shortcomings. For example, one is to use a larger power amplifier, which can transmit the communication signal with its operating point substantially backed-off substantially from its compression point (the upper end of the linear region). This can carry costs in power and heat.
A modification of the larger amplifier technique described above is to use a transmitter power amplifier having just slightly above the transmission power requirement, and to set its operating point as close as possible to the maximum point before distortion becomes unacceptable. Costs, though, include a reduced safety margin for operating point drift.
Another conventional technique is to closely control the level of the input feed to the amplifier. However, even if the drive level is measured accurately, and kept reasonably constant, amplifier characteristics can change, e.g., over temperature and frequency. Compensation, e.g., temperature monitor circuitry in the transmitter, can be applied, but can have undesirable overhead costs.
Another conventional technique directed to reducing adjacent channel interference is to monitor power output of transmitter power amplifier. However, the monitoring can add significant hardware to the amplifier, and can degrade its operation. Also, due to fabrication variances, different samples of the power amplifier can exhibit different power versus distortion characteristics.
In addition, there are conventional techniques for directly measuring adjacent channel interference, including at a receiver. However, conventional measurement techniques can be computationally intensive and can require interruption of system operation.
Accordingly, what is needed is a method to reliably measure amplifier operating point without requiring special instrumentation on the transmit side, and without being computationally expensive.